A major cause of serious battlefield injuries during Operations Iraqi Freedom and Enduring Freedom has been the use of improvised explosive devices against coalition troops. Maxillofacial, head, and neck regions are particularly at risk for injury and pose distinct treatment challenges to clinicians (1, 2). These combat associated traumatic wounds are different from injuries experienced in the civilian sector due to massive tissue damage from extremely high velocity, high-energy projectiles, involvement of blast wave effects, and a higher rate of wound contamination from the environment (2). Previous studies indicate a significant percentage of these wounds were infected with multi-drug resistant bacteria such as methicillin resistant Staphylococcus aureus (MRSA) (1). In addition to those injured in combat, military recruits are one of the groups identified as at risk for acquiring MRSA infections. This is notable because those affected are typically young, healthy individuals without any apparent risk factors, and these infections have been associated with an increased incidence of hospitalizations. Because emergence of multi-drug resistant bacterial infections is a growing problem in military and civilian populations worldwide, novel anti-microbial therapies are needed as alternatives to traditional antibiotic regimens.
Current treatment regimens for bacterial infections focus on use of antibiotics. The challenges associated with the successful treatment of microbial infections are increasing because the rate by which bacteria develop resistance to current treatment modalities outpaces the development of new antibiotics. S. aureus is the most common pathogen isolated from patients, and methicillin resistant strains now account for approximately 60% of S. aureus isolates in intensive care units in the US (3). Vancomycin is commonly used to treat serious MRSA infections because most strains of the pathogen exhibit resistance to many other classes of antimicrobials. However, cases of MRSA with reduced susceptibility or resistance to vancomycin have begun to emerge in hospitals and are associated with increasing patient mortality (4). The continued development of bacterial resistance indicates an urgent need for treatment approaches that do not rely solely upon antibiotics.
One approach being tested by several groups is photodynamic therapy, which uses light absorbing dyes to generate toxic oxygen radicals to kill the bacteria. However, this treatment might not be effective for infections in hypoxic wound environments (5). Another promising approach is to use metal nanoparticles, and laser energy to physically damage the bacteria.
The optical properties of conductive metal nanoparticles (NPs), such as those made of gold and silver have been associated with the surface plasmon resonance (SPR) of metals, which when confined to small colloids, is referred to as the localized surface plasmon resonance (LSPR). This phenomenon, in which the free electrons oscillate collectively on the metal surface when irradiated with particular energies of light, causes wavelength dependent absorption and scattering of light, and is the source of the colors associated with metal nanoparticles. The size, shape, and composition of the colloidal particles determines the energy of the SPRs, and therefore, control over the synthesis of metal NPs provides an ability to tune the optical properties of the nanometals contained therein.
Metal nanoparticles, due to their relative inertness, sub-100 nm size, unique electromagnetic properties, and strong optical tunability, have attracted attention in the biomedical field. For example, because SPRs enhance many optical processes, including Raman scattering, fluorescence, and two-photon excited luminescence, gold NPs have been used in optical diagnostics and as contrast agents for bioimaging. When gold NPs absorb light energy, they also release heat, making them useful in photothermal therapy applications targeting cancer and bacterial cells. Laser-induced photothermal phenomena induce physical disruption of the bacterial cells leading to death. This is a different type of killing mechanism than that caused by antibiotics or photodynamic therapies that induce chemical damage via generation of oxygen radicals. Resistance to photothermal destruction has not been reported in the literature.
Despite the prospect of biomedical utilizations of metal NPs, the use of metal NPs for medical diagnosis and treatment is limited, because NPs cannot be fully integrated into the biological realm without changes to their surface chemistry. Biomolecules interact with cells through a multitude of chemical interactions and physical forces. The interactions between biological systems and metal NPs, on the other hand, are non-specific. In order to realize the full biomedical potential of metal nanoparticles, the nanoparticles must interact specifically with biological matter, including cell surface components. At the same time, nanoparticle aggregation and nonspecific interactions with molecular and cellular constituents of the biological system must be minimized. Thus, there is a need in the art for metal nanoparticles that can be readily modified to precisely control their electromagnetic and biofunctional properties.
Zharov et al. taught a method using gold nanospheres and pulsed laser irradiation to induce a photothermal effect for bacterial destruction (5). This method involved a two-step process to bind the particles to the bacteria, where bacteria were first incubated with primary antibody against S. aureus Protein A in the cell wall then incubated with gold nanospheres coated with a secondary antibody against the primary antibody. However, the Zharov et al. (5, 7) only tested the technique against methicillin sensitive bacteria. The method's effectiveness against drug resistant bacteria is not addressed. Furthermore, the level of expression of Protein A, which is the targeted protein of Zharov study, has been reported to vary among different strains of MRSA and among the different phases of growth of the bacteria (6). As a result, the Zharov method is likely to be ineffective against strains or phases of the bacteria that fails to express Protein A.